CHEM*2700: Experiment #4
Isolation of eugenol
Important:
In experiment #2, we used a heating mantle controlled with a Variac as our heat source for the distillation. In this experiment, we’ll use a similar set-up to provide heat for the initial steam distillation.
So don’t plug the heating mantle directly into the wall. Plug it into the Variac.
In this experiment, we’ll need to use a stir bar in order to stir and agitate the clove/water suspension, to prevent the solution from becoming too violent.
Water: density = ~1.0 g/mL; boiling point = 100 ˚C
DCM: density = 1.3 g/mL; boiling point = 39.6 ˚C
There’s a flow chart below, which will help you know which layers to keep in the extraction.
Steam distillation
A mixture of clove powder and water results in an insoluble suspension. This immiscible system can be simply described by:
- Here, total pressure is equal to the sum of its partial pressures.
- As we heat this system, each component independently exerts an increasing vapour pressure as a function of temperature.
- Boiling occurs when the total pressure exceeds atmospheric pressure. Due to the additive nature of this system, total pressure will exceed atmospheric pressure at a lower temperature.
Distillation Apparatus
Notes:
- In this diagram, the stir plate, heating mantle, and Variac aren’t shown.
- Despite what’s shown in the lab manual, we’ll only use two clamps. One to hold the round bottom flask, and another to hold the collection flask.
- Don’t forget to place a magnetic stir bar into the round bottom flask containing the clove/water suspension.
- Again, note the positioning of the thermometer bulb to accurately measure the temperature of the vapours.
Liquid/liquid extraction
The video above shows to use a separatory funnel. Pay attention as to which layers are kept. However, we will not be shaking the funnels. We will do the mixing in an Erlenmeyer flask, pour the liquids back into the funnel, and continue the separation from there.
Eugenol extraction
After collecting the distillate, we’ll be using liquid/liquid extraction in order to isolate eugonol. The initial distillate will contain organic compounds in water.
Understand the purpose of each extraction, how we manipulate pH in order to isolate eugenol, and how to separate the differnt layers.
Misunderstanding can lead to the discarding of the wrong layer, and/or improper modification of layers.
pH and pKa relation
Extraction #2 will leave us with a basic aqueous layer with eugenol in its deprotonated state, while the eugenol acetate, which has no protons, which can be removed given the base treatment, will remain in the organic DCM layer. Ideally, we can re-acidify this basic aqueous layer to provide us with eugenol in its protonated form, and upon extraction with DCM, we can isolate pure eugenol.
The phenolic structure of eugenol allows us to estimate that it will have a pKa of about 10.2.
- Although lowering the pH of the solution would result in a larger concentration of protonated neutral eugenol, highyl acidic environments can degrade the compound, namely the allylic group of eugenol (think alkene reactions and acid-catalyzed hydration).
- However, this shouldn’t a problem if you follow the instructions properly.
FYI Decaf in a nutshell
This concept of selective extraction has been applied to the decaffeination processes. Although caffeine is the main stimulant in coffee, there are thousands of other chemicals that contribute to its taste (e.g. sucrose, various proteins, citric acid, tartaric acid, and other ‘flavour’ molecules).
Consider caffeine; its polar nature allows to to be soluble in water, yet its organic make-up allows for greater solubility in common organic solvents (DCM, EtOAc, etc.). So using hot water to water to extract caffeine from raw green coffee beans would be a futile effort. We’d be left with flavourless (albeit decaffeinated) coffee beans, and an aqueous extract that contains caffeine, and everything else that makes coffee great.
However, treatment of raw green coffee beans with organic solvents would result in an extraction of caffeine with minimal extraction of the flavouring components due to caffeine’s high solubility in these solvents. Solvents such as benzene, chloroform, dichloromethane, and ethyl acetate have been used to selectively extract caffeine. This solvent extraction method, does present an obvious drawback: the presence of organic solvents in consumable goods. Although these solvents are relatively volatile, and get reduced to residues in the parts per million range, “green” solventless methods have replaced this solvent extraction method.
A more recent, common practice is to use carbon dioxide in its supercritical state (sCO2); CO2 at a temperature and pressure above its critical point, where the distinct liquid and gas phases don’t exist (i.e. sCO2 is able to have gas-like free-flow properties, while retaining liquid-like solvent properties). sCO2 can effuse through the raw coffee beans and dissolve caffeine while leaving the other components undisturbed. This method has a relatively low toxicity, can be easily removed, and carries a reduced impact on the environment when compated to organic solvents. However, this method requires pressuring CO2 above 72.8 atm, which makes it rather expensive, and is typically reserved for larger, inexpensive batches of coffee.
The popular Swiss water method creatively uses water to selectively extract caffeine form coffee beans. Imagine a batch of raw green coffee beans, which is treated with hot water. This would result in the aqueous extraction of caffeine along with the other flavourful components of the coffee beans, called the green coffee extract (GCE). This GCE is then passed through a charcoal filter, which removes only the caffeine from the GCE, while leaving the flavour components behind, resulting in a caffeine-free, yet ‘flavour’-saturated GCE.
This GCE can be used to extract caffeine from another batch of raw green coffee beans, without extracting any flavour components of this new batch of coffee beans, since the GCE is already saturated with “flavour”. This leads to decaffeinated raw green coffee beans, with minimal losses in flavour.
